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Beware Of Humans

In a normal year, on a spring night in a Midwestern field, Joy O’Keefe would raise a banner of fine mesh stretched between wooden poles 30 feet in the air. Fluttering in the twilight like a lunch lady’s hairnet gone to war, the mesh would be a snare for O’Keefe’s quarry. 

“We like to say we only catch the stupid bats,” she told me — the ones that fly headlong into this obvious trap and don’t immediately bite their way out afterward. These captured bats teach scientists about the health and well-being of North American bats more broadly. Catching them, measuring them, gluing tiny radio transmitters to their freshly shaven backs — all this is crucial to protecting animals whose numbers are in steep decline, and whose continued existence is vital to human agriculture. 


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But the fields were empty in March 2020, and the bats — the brilliant and the dimwitted alike — sailed peacefully through the evening skies. Instead, it was the humans who were trapped — on a massive conference call. COVID-19 had come to America, and scientists already suspected the virus that caused it, SARS-CoV-2, had originated in bats before making a leap to humans. Nobody knew what that might mean for North American bats, though, and so more than 100 epidemiologists, bat researchers, wildlife managers — anyone who might be able to add clarity to a deeply uncertain situation — dialed in to a call to discuss what to do. The stakes were high, time was short and the scientists knew this: The bats would need our help.

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“In North America we pose a far greater risk to our bats than they do to us,” said O’Keefe, a bat ecologist and professor of environmental science at the University of Illinois at Urbana-Champaign.

ILLUSTRATION BY EMILY SCHERER / GETTY IMAGES

COVID-19 is a zoonotic disease, an illness that jumped from animal hosts to humans. But disease transfer isn’t just a one-way street. It takes only a bit of evolutionary bad luck to turn a bat’s head cold into a human’s killer. But it takes only a little more for the same virus to jump from humans to other animals. Zoonosis begets reverse zoonosis, which can, in turn, come back around to zoonosis again. A virus we give to a bat could, someday, come back around to reinfect us. Animals’ health is ours, ours is theirs, theirs is ours. 

For North American bats, contracting this new virus carried the risk of absolute annihilation. Unlike Old World bats, they aren’t natural reservoirs for the branch of the coronavirus family that includes SARS-CoV-2 (the official name of the novel coronavirus), so they have no preexisting immunity to it. And spring was a crucial time. COVID-19 reached the U.S. just as bats were waking from winter hibernation and humans were gearing up for bat research season. The people on that March conference call knew that, within weeks, human researchers would begin catching bats — weighing them, measuring them, literally breathing directly into their tiny faces. We barely knew where COVID-19 had come from, and already we had to think about what it might spread to next.

The bats weren’t — and still aren’t — the only animals of concern. Even now, a year later, researchers are still working to figure out which animals, in which places, are most at risk. It’s a question that has consequences for both animals and people. Every new species that becomes a host for COVID-19 is also a population where the virus can change, mutate, and boomerang back to us. Bats are a bellwether, an example of how our relationships with animals can threaten both our health and theirs. 

Andreas Arnold / picture alliance via Getty Images

There’s an intimacy to studying bats. But there’s also an industrial detachment. The individual creatures pulled from Joy O’Keefe’s nets must go through a series of medical tests and sampling. Blood is drawn, hairs are plucked. Genitals are inspected and bats are sorted into spreadsheet cells for males and females. To figure out which females have recently given birth, the scientists blow lightly on the bats’ soft, furry chests, a trick that makes their nipples pop out and milk begin to trickle.

But to comply with animal welfare regulations, all this work must be packed into less than 30 minutes. For the sake of speed and efficiency, the bats become widgets in a small mammal assembly line, one human after another holding them, prodding them, breathing above and on them. 

In spring 2020, nobody knew whether North American bats could contract SARS-CoV-2 from humans who were studying them this way. But after the bat signal went up in March, the U.S. Fish and Wildlife Service decided not to take a chance and put a pause on the fieldwork it funded. “We weren’t fully sure what the risks were, but we knew what the consequences could be,” said Jeremy Coleman, a wildlife biologist at the USFWS who coordinates the agency’s response to white-nose syndrome, a deadly fungus that has killed at least 7 million bats in North America since 2006. 

Coleman knew those risks because of his job. Researchers had figured out a little more than a decade ago that scientists themselves were spreading white-nose syndrome from bat colony to bat colony. Full of good intentions but lacking gloves, researchers had become partly responsible for the epidemic. Obviously a horrific loss for the bats themselves, it has also affected humans. That’s because bats are an important part of our food supply, eating insects that would otherwise attack crops and pollinating some of our favorite fruit trees.

White-nose syndrome has killed at least 7 million bats in North America since 2006. Humans have helped spread it, worsening the crisis.

Carolyn Cole / Los Angeles Times via Getty Images

That realization changed how research like O’Keefe’s is done. After that discovery, she began to wear gloves, changed them between handling each bat and boiled her teams’ gear and clothes in a daily dance of decontamination. Reverse zoonosis put the fear of God into the bat research community. 

But, more broadly, reverse zoonosis is still a risk that isn’t taken as seriously as it should be, scientists told me. It’s probably more common for viruses to spread from humans to animals than we even know, said Kevin Olival, vice president for research at the nonprofit EcoHealth Alliance. And a virus that does this is a virus you can’t eradicate. It becomes endemic, waiting patiently, changing in ways that can make it able to infect us again.

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Take influenza, a virus as familiar to us as a nosy neighbor, and with the potential to be every bit as obnoxious. When this virus mutates in ways that make it, for a season, a more serious threat, the very names we call it highlight the role animals played in that transformation — swine flu, bird flu. But while it’s true that animals were the hosts while various strains of flu swapped genetic information and mutated, the resulting new strains emerged from animals only because humans had passed precursor strains to those creatures to begin with. 

An industrial pig farm, for example, provides ample opportunity for transmission between humans and animals. The strain of influenza that launched the 2009 swine flu pandemic came from farms in Mexico, said Martha Nelson, a staff scientist with the U.S. National Institutes of Health’s Multinational Influenza Seasonal Mortality Study. At the time, it wasn’t found in domestic pig populations anywhere else in the world, but today you can find the descendent strains of that pandemic in pigs everywhere on earth that pigs are farmed. That’s not because the pigs had gone globe-trotting. “Those human viruses have gone back into the pigs and reassorted with endemic swine viruses to create these new [strains], and one of the problems is, that really increases the genetic diversity of influenza in pigs. And some of [those new strains] have proven capable of infecting humans,” Nelson said. Since 2011, these new strains have infected more than 400 people, mostly kids who raise pigs for county fairs in the United States. Zoonotic disease is not a straight line; it’s a flat circle.

Even camels need a nose swab sometimes. In this instance, a camel in Kenya is tested for MERS.

TONY KARUMBA / AFP via Getty Images

But while experts are fairly certain that SARS-CoV-2 came from Asian bats, that knowledge doesn’t necessarily tell them which animals humans are likely to spread the virus to. Figuring that out has involved a lot of trial and error

Camels and alpacas, for example, are known carriers of MERS, COVID-19’s more deadly cousin, but they don’t seem to be susceptible to SARS-CoV-2 at all. Bats are thought to carry thousands of coronaviruses, but some bats seem to be susceptible to SARS-CoV-2 while others (like the big brown bat, common to North America) aren’t.


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Meanwhile, cell tests on pigs made it seem, at first, as though they could very likely contract the virus. Biologically, SARS-CoV-2 is able to infect vertebrate animals that carry a receptor on the surface of their cells called ACE2. This receptor is basically a lock, and the virus is the key that fits. In a test tube, ACE2 opened pig cells to SARS-CoV-2 like a welcome friend. But it turns out that a pig is more than the sum of its bacon bits.

“The pigs, they do have a good receptor, but the expression of this is different in humans,” said Jürgen Richt, director of the Center of Excellence for Emerging and Zoonotic Animal Diseases at Kansas State University. The ACE2 receptor was expressed in the kidney and testicle cells used in the lab. “But not so much in the respiratory tracts,” he said. “That’s why the cell lines are receptive, but the pigs aren’t.” 

Some higher-risk species have emerged from the research, though. 

Many animals are at risk of contracting SARS-CoV-2

Animal species for which information on natural or experimental SARS-CoV-2 infection is available, by evidence of transmission

Farmed animals Infection Documented Transmission
Species Susceptibility Occurred in nature? within species? to humans?
American mink High
Ferrets High
Raccoon dogs High
Rabbits High
Pigs, cattle Extremely low
Poultry None
Companion animals Infection Documented Transmission
Species Susceptibility Occurred in nature? within species? to humans?
Domestic cats High
Hamsters High
Dogs Low
Wildlife Infection Documented Transmission
Species Susceptibility Occurred in nature? within species? to humans?
Large cats High
Fruit bats High
Gorillas High
White-tailed deer High
Marmosets High
Macaques High

Source: World Organisation for Animal health

Minks — animals that have already demonstrated in the real world that they can contract SARS-CoV-2 from humans and then spread it back to us — and their relatives, such as ferrets, are vulnerable. Rabbits and domestic cats are both highly susceptible. So are deer and gorillas. In general, the animals that have turned out to be the biggest risks for reverse zoonosis of COVID-19 are more like bats than like pigs. They’re not creatures of mass agribusiness. Nor are they animals that are safely off somewhere living free from moments of intense human contact. Instead, they’re somewhere in between. And that … well, it’s not great news.


Nobody loves vampire bats, but we can learn a lot from them. Like the name implies, these are the only bat species that actually drink blood. Worse, they can spread rabies to the valuable cattle they feed on. While other bats are benefiting from elaborate conservation efforts, multiple governments are actively trying to kill as many vampire bats as possible, said Gerald Carter, a professor of evolution, ecology and organismal biology at Ohio State University whose research focuses on vampire bats.

But culling vampire bats doesn’t actually reduce the number of rabid cows. In fact, Carter said, the places that kill more vampire bats end up with higher rates of rabies in livestock. That’s because some bats that were already resistant to rabies and weren’t spreading it are inevitably eliminated, leaving an ecological niche wide open for other, rabies-carrying bats to fill. 

“One problem we’ve faced perennially with controlling wildlife diseases is that we’ve learned we aren’t particularly good at it,” the USFWS’s Coleman told me. And that’s why the animals that seem most susceptible to COVID-19 make researchers nervous — they present challenges that, ironically, wouldn’t exist if the virus were targeting domestic species that are constantly in close contact with us. 

A respiratory virus on a farm is a bad thing, Richt said, but once you know it’s there, it’s easy to contain. There’s a market for vaccines for pigs, and you can line the pigs right up and give the vaccines out. If that fails, worse comes to worst, you can kill all the animals in a given barn, or a farm. Nobody wants to, but it’s an option. Wild animals are another thing entirely. When a virus like SARS-CoV-2 gets into wildlife, what are you going to do? Who is going to pay to vaccinate the wild bats? Logistically, how do you do it?

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And the animals that seem to be the most susceptible to contracting COVID-19 aren’t just wild. They’re a little halvsies, with populations both in and out of human contact. Gorillas that live in the forest but depend on ecotourism for survival. Deer and mink that are both farmed and hunted wild. The tens of millions of feral cats that live together under America’s sheds and broken-down Camaros, hesitantly poking their heads out for the kindness of strange humans who will feed and pet them. Even the North American bats, really and truly wild, aren’t free from significant human interaction thanks to those biannual research seasons

Carolyn Cole / Los Angeles Times via Getty Images

Another problem is we just flat out don’t know where all the wild animals are. These are creatures that live and behave and interact in ways we often have no knowledge of. The northern long-eared bat was an extremely common species in North America prior to the apocalypse of white-nose syndrome. People ran into this bat everywhere. We knew a lot about this bat. But we never did figure out where all of them spent the winter, O’Keefe, the bat ecologist, told me.

Wild or semi-wild populations of animals give a virus plenty of new hosts — and new contacts to infect. Circulating in a population of wild animals, the virus will mutate. Some mutations will thrive and others won’t; the normal and natural process of evolution will happen. Then some of those mutations will make for a virus that can infect whole other species of animals that weren’t susceptible before. Viruses don’t just stay where you last set them down. 

We can already see this happening. SARS-CoV-2 couldn’t infect mice when it was originally identified back in early 2020. But SARS-CoV-2 as it currently exists can. Somewhere along the line, the virus changed in such a way as to make mice susceptible. This is the kind of story that gives North American bat researchers pause, even though there’s not yet a lot of evidence that the bats on this continent can catch COVID-19. Over the course of the winter, the USFWS and the U.S. Geological Survey — both of which are responsible for large portions of America’s bat science — conducted a more rigorous evaluation of the possibility that human researchers could pass COVID-19 to their bat subjects. The study concluded that if no precautions were taken, the chances of a team of five scientists infecting at least one bat in the course of their work was 1 in 1,000.

Based on that, in-person bat research will resume this spring. Scientists will go back to those twilight fields, and bats — at least, the dimmer ones — will return to their caves with new tales of alien abduction and bizarre medical procedures. 

But risk remains. When the press release announcing this study came out, it read “Low Risk of Researchers Passing Coronavirus to North American Bats.” That did not sit well with Coleman. “I have to admit, I’m not real excited about the title that went out with that press release,” he told me. “I think this is something that is still of concern.” That’s because the 1-in-1,000 risk is a snapshot. It captures a moment in time when, as far as we know, no North American bats are particularly susceptible, and no mutations have made any bat more susceptible than it used to be. It doesn’t account for change. It doesn’t even account for the way different diseases can multiply the risks of one another. White-nose syndrome, for example, is known to reduce the immune response of bats and make them more susceptible to diseases they otherwise might be safe from. Nobody knows yet what effect that could have on the bats’ susceptibility to COVID-19, O’Keefe told me. 

But none of this is the bat’s fault. It didn’t choose to fly into our nets — it’s just not really great at avoiding them. The problem is not that animals are dangerous carriers of disease that need to be controlled, experts said. The problem is that humans can’t seem to stop getting all up in animals’ business in a wide variety of ways, any of which can spread disease — from us to them, or them to us. 

In February, the Centers for Disease Control and Prevention published its guidelines for handling wild animals. These are presented in the form of a pyramid. At the top, the best thing you can do to prevent the spread of SARS-CoV-2: Just don’t handle the animals at all. At the bottom, if you can do nothing else, make sure you wear some protective equipment, such as masks.

In the wake of that document, the bat research community has begun to sound a little like a bunch of newly vaccinated humans trying to decide if and when they can have brunch with an unvaccinated friend. The risk is low. But is it really necessary? If we wear masks, is it fine? The answers depend on whether you think wearing a mask is great protection or just the bare minimum you need to do if there’s no other way to avoid contact. “I guess I don’t really feel like it’s solved,” O’Keefe said. “The knowledge we’ve gained has told us it’s possible for us to be transmitting to wild animals we haven’t even contemplated and probably should be giving it more attention. The precautions we’re taking, we should have probably been doing that all along.” 

These are hard choices. They force scientists to face the equally firm reality that observing a thing also involves changing it. The act of catching a bat isn’t a neutral choice, with no consequence other than the collection of data. Keeping an animal in a zoo, or a wildlife preserve, or under your dad’s old car is not neutral either. We keep thinking of our lives as separate from those of wild animals, even when we are actively touching them. We like to pretend that “wild” and “domestic” are hard and fast lines and that there are places animals can live where they aren’t affected by us. 

When these delusions catch up with us, we cull the animals. But, said Angela Bosco-Lauth, a professor of veterinary medicine specializing in infectious disease at Colorado State, the truth about zoonosis is that these viruses expose the stark reality of which species you really have to worry about: It’s us. “They aren’t our problem,” she said. “We’re theirs.”


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